In the field of dental materials, the conversion from metallic materials to polymeric materials has been proceeding rapidly in recent years. Such dental materials include, for example, surface coating agents such as surface glazing agents and hard coating agents; composite resins such as filling composite resins, facing hard resins, composite inlays and jacket crowns; artificial teeth, denture bases and denture base liners; and orthodontic materials such as special impression tray materials and brackets.
These dental materials usually comprise a (meth)acrylate monomer and a polymerization initiator. Moreover, if need arises from the intended purpose, they can additionally contain inorganic materials, (meth)acrylate polymers, pigments, solvents, polymerization inhibitors, oxidation stabilizers and other additives. Typically, they are cured by photopolymerization by means of visible light or ultraviolet radiation, by thermal polymerization, or by redox polymerization. Moreover, the curing is often performed in the mouth.
In polymerizing these dental compositions, however, it is difficult to cause all double bonds to participate in the polymerization, so that some double bonds tend to remain. Moreover, when the polymerization is performed in the mouth, the presence of air cannot be avoided. Thus, the polymerization inhibiting effect of oxygen develops a tendency for a large amount of double bonds to remain, especially on the surface of the cured product. When a large number of double bonds remain on the surface of the cured product, the growth of bacteria on the surface of the cured product is promoted because of the presence of active sites comprising carbon-to-carbon double bonds. Thus, after the cured product has been used in the mouth for a long time, plaque resulting from bacterial contamination adheres to the cured product, causing a marked reduction in surface gloss and smoothness to impair its appearance and also posing a problem from the viewpoint of hygienics.
In order to minimize the polymerization inhibiting effect of oxygen, a number of methods using a water-soluble polymer (such as polyethylene glycol, polyvinyl pyrrolidone or polyvinyl alcohol) as an air barrier have been proposed, for example, in Japanese Patent Laid-Open Nos. 134705/'84 and 100505/'85. In these methods, an unpolymerized dental composition is applied, filled or formed into a desired shape, covered with the above-described polymer, and cured in the state shielded from oxygen. However, this is not very effective in preventing double bonds from remaining on the surface of the cured product, and cannot be regarded as a satisfactory measure to cope with the adhesion of plaque.